1. Field of the Invention
The present invention relates to a piston assembly for a radial piston hydraulic motor.
2. Description of the Related Art
The above mentioned piston assembly is disclosed e.g. in GB 2,064,700, FR-A 2,368,619 and WO 98/14722. It has been found that none of the said solutions are satisfactory, neither in the sense of the production of an engine with minimum overall dimensions nor the minimum load of the piston assembly, nor in combination with each other.
It is therefore the primary object of the present invention to propose a piston assembly for a radial piston hydraulic motor which will enable minimum overall dimensions with the maximum conversion of hydraulic energy into mechanical energy and vice versa, during the entire life span of the motor, and with the least amount of energy lost.
Another object of the present invention is to create a piston assembly where the surface pressure will be distributed as equally as possible over the surface of the portion of the piston which is in direct contact with the wall of the working cylinder.
Yet another object of the present invention is to create a piston assembly which is prevented from rotating about the longitudinal axis thereof.
According to the invention this problem is solved by a one-piece piston formed as a cylindrical body with a crown attached to it. The said piston crown has a variable diameter so that the outline thereof represents at least piecewise smoothly decreasing curve when observed in the direction away from the piston body and towards the middle thereof. The crown is preferably formed as a spherical segment, it may, however, comprise different shapes e.g. of a frustum of a cone. A bearing bush inserted into the said piston comprises a roller bed and a roller rotationally arranged in the said bed. The longitudinal axis of the roller essentially coincides with the transverse axis through centre of gravity of the cylindrical piston body. The inner portion of the piston body, which is situated below the longitudinal axis of the roller, is formed with a decreasing diameter in the direction away from the piston crown. In order to allow the roller to be placed into the bed of the bearing bush, the said inner portion of the piston body is provided with a number of grooves distributed circumferentially around the inner portion of the piston body. Said grooves extend from the lowermost edge of the piston longitudinally towards the piston crown. Moreover, said grooves coincide with the edges of the roller.
With such an arrangement of the roller with respect to the cylindrical piston body the surface pressure is distributed as equally as possible over the friction area of the piston, resulting in considerably lower wear of the surface of the piston which is in direct contact with the cylinder wall.
According to the invention, the bearing bush of the piston assembly is formed in a way that the portion thereof facing the piston crown is congruent with the inner surface and the roof of the piston, respectively. A bore is formed in the said cylindrical portion of the bearing bush, said bore extending perpendicularly to the longitudinal axis of the piston and is intended to receive the roller. The bearing bush is secured by means of an adhesive joint on the contact surfaces with the piston, against the rotation in an axial direction of the piston assembly.
The inner surface of the piston and/or the said inner portion of the piston body is preferably shaped spherically, said fictive sphere which forms the said spherical zone comprises a diameter which circumscribes the roller. In this way the bearing bush can still be placed inside the piston and, after the bearing bush and roller are rotated about the longitudinal axis of the piston, the roller is at the same time prevented from falling out of the piston.
The bearing bush and the roller therewith are secured against rotation about the longitudinal axis of the piston by means of an adhesive joint between the inner surface and the roof of the piston, respectively, and the bearing bush, the latter also being secured against longitudinal displacement.
In order to prevent the piston assembly from rotating about the longitudinal axis thereof, the piston crown and the bearing bush are provided with a receiving bore, extending in a longitudinal direction of the piston assembly, in which bore there is fixed a guide pin which, on the other hand, is guided with some play in a guide bore made in a body of the hydraulic motor. Here, the guide pin is formed with a longitudinal bore and a transverse bore, the latter lying outside the said receiving bore. The bearing bush is provided with a connecting bore mutually linking the roller bed and the lower i.e. base end of the receiving bore. The lubricant, i.e. a hydraulic oil in a given case, passes through the connecting bore into one or more of lubrication grooves provided in the roller bed of the bearing bush. This way an additional forced lubrication of the roller is facilitated.
The invention will be more readily understood on reading the following description with reference to the accompanying drawings, in which: